1. Field of the Invention
The present invention relates to an optical signal detecting circuit, and particularly to an optical signal detecting circuit suitable for performing optical pickup in an information reproducing device.
2. Description of the Related Art
An optical disk, typified by a digital versatile disk (DVD), has information recorded on its information recording surface in lines of minute marks (or pits). The recorded information is reproduced as an RF signal (reproduction signal) which is obtained as follows. A laser light is condensed by an objective lens and applied onto the information recording surface. A change in the intensity of the light reflected is detected based on the differences in reflectivity between the marks and spaces between the marks. The change thus detected is converted into the RF signal. In reproducing information, it is necessary to cause a condensed point of the laser light to accurately follow the mark lines on the information recording surface so as to accurately detect the minute pits. To cause the condensed point to follow the mark lines, the position of the objective lens is adjusted as follows. Specifically, an optical means detects a positional shift of the condensed point from the information recording surface in an optical axial direction as well as a positional shift of the condensed point from the mark lines in a radial direction of the disk. These positional shifts are converted into electronic signals called a focus error signal and a tracking error signal, respectively. These signals are then fed back to adjust the position of the objective lens.
FIG. 2 shows a typical configuration of an optical signal detecting circuit that converts optical signals into electronic signals in an optical disk device. A quadrant photodiode is used as a light receiving element. Currents I1, I2, I3, and I4 flow though photodiodes 201, 202, 203, and 204, respectively. Here, the currents I1, I2, I3, and I4 have magnitudes proportional to the intensities of light received by the respective photodiodes 201, 202, 203, and 204. The currents I1, I2, I3, and I4 are converted into voltage signals V1, V2, V3, and V4 by current-to-voltage converters 211, 212, 213, and 214, respectively, each consisting of an operational amplifier and a feedback resister. As shown in the following equations, each current-to-voltage converter outputs a voltage proportional to the current flowing through the corresponding photodiode.
                    {                                                                              V                  1                                =                                  RI                  1                                                                                                                          V                  2                                =                                  RI                  2                                                                                                                          V                  3                                =                                  RI                  3                                                                                                                          V                  4                                =                                  RI                  4                                                                                        (        1        )            , where R is the value of the feedback resister of the current-to-voltage converter. Typically, the frequency response characteristics of each photodiode are improved by reducing the terminal capacitance of the photodiode through application of a reverse bias voltage thereto. In the configuration example shown in FIG. 2, each photodiode is reverse-biased since, due to the virtual short characteristics of the amplifiers, a potential of the cathode of the photodiode which is connected to the inverting input terminal of the corresponding operational amplifier is equal to a positive reference potential Vref applied to each of the non-inverting input terminals.
A focus error signal (FE), a tracking error signal (TE), and an RF signal (RF) are generated by performing addition and subtraction on V1, V2, V3, and V4 as shown in the following equations.
                    {                                                                              F                  ⁢                                                                          ⁢                  E                                =                                                      (                                                                  V                        1                                            +                                              V                        3                                                              )                                    -                                      (                                                                  V                        2                                            +                                              V                        4                                                              )                                                                                                                                            T                  ⁢                                                                          ⁢                  E                                =                                                      (                                                                  V                        1                                            +                                              V                        4                                                              )                                    -                                      (                                                                  V                        2                                            +                                              V                        3                                                              )                                                                                                                                            R                  ⁢                                                                          ⁢                  F                                =                                                      V                    1                                    +                                      V                    2                                    +                                      V                    3                                    +                                      V                    4                                                                                                          (        2        )            
Assume that an astigmatism method and a push-pull method are employed as a focusing control method and a tracking control method, respectively. The optical signal detecting circuit having the quadrant photodetector as described above has an advantage of simplifying a reproducing optical system because of its capability of providing signals necessary for reproducing information with only a single detection system without having separate detection systems for servo and for providing an RF signal.
Japanese Patent Application Publication No. Hei 3-285408 (referred to as Patent Document 1 below) describes a light detecting circuit. This light detecting circuit includes four photodiodes combined in a quadrilateral form, two amplifiers each serving as a current-to-voltage converter, two voltage sources, and two current detecting resisters. The cathode of the first photodiode is connected to the cathode of the second photodiode, and their connection point is connected to the inverting input terminal of the first amplifier. The anode of the second photodiode is connected to the cathode of the third photodiode, and their connection point is grounded via the second voltage source and the second current detecting resister as well as being connected to the non-inverting input terminal of the second amplifier. The anode of the third photodiode is connected to the anode of the fourth photodiode, and their connection point is grounded via the first current detecting resister as well as being connected to the non-inverting input terminal of the first amplifier via the first voltage source. The cathode of the fourth photodiode is connected to the anode of the first photodiode, and their connection point is connected to the inverting input terminal of the second amplifier.